1. Field of the Invention
The present invention relates to a low-power alternator, that is, an alternator having a power ranging between 0.1 and 100 watts, typically from 1 to 3 watts. Such alternators are especially used for the bicycle lights or in flashlights recharged by means of a crank.
2. Discussion of Prior Art
Alternators, that is, electric generators providing a D.C. voltage, are here considered. Alternators comprising a rotor formed of a permanent magnet and a stator formed of a winding are more specifically considered. Conventionally, when such an alternator is desired to be formed, it is provided, to optimize the concentration and the use of the magnetic field generated by the rotor, to associate the stator windings with soft iron magnetic cores.
Because of the provision of magnetic cores at the stator, the alternators have a significant no-load running torque and a non-negligible weight. Thus, alternators used for bicycles, improperly called “dynamos”, generally have very poor efficiencies. For example, a conventional “dynamo”, provided to rotate at 3,750 rpm for a bicycle speed of 15 km/h and capable of providing a useful electric power on the order of 3 watts has losses on the order of 6 watts. When the alternator is not connected, the losses are on the order of 7 watts. The mechanical power to be provided thus ranges between 7 and 9 watts. The losses are far from being negligible with respect to the total effort provided by a cyclist pedaling at a normal pace. Indeed, it can be considered that with a bicycle of good quality, a cyclist pedaling at a normal pace to have a 15-km/h speed on flat ground must provide an effort on the order of 70 watts. When the alternator is used, in full load or no-load mode, the cyclist must provide an effort greater by 10% than his normal effort. As a result, the cyclist has to place his alternator in a non-operating position when he does not need it, which requires providing mechanical parts intended for the actuation or not of the alternator, which for example comprises a roller rolling on a wheel of the bicycle. To avoid this additional effort, the cyclist may delay his use of the lighting of his bicycle while the night is falling and thus jeopardize his security.